Factors that mediate changes in physical activity are important to identify, for purposes of informing effective public health interventions. While current knowledge is limited, mediators of physical activity are likely physiologic, psychosocial, and environmental in nature, and may also be modified by certain factors, leading to disparities in health outcomes. Life stages and transitions through these stages may be effective points to intervene and to promote preventive health behavior. Pregnancy is one such triggering life event, since women experience rapid weight gain during pregnancy and subsequent weight loss during the postpartum period that influences changes in activity. This study proposes to (i) identify whether changes in physiologic, psychosocial, and environmental mediators are associated with changes in physical activity from pregnancy to postpartum and to (ii) examine whether changes in physical activity during this time are moderated by sociodemographic, health, and neighborhood measures, among a cohort of women. To accomplish this, a comprehensive neighborhood research component will be added to an ongoing prospective study of pregnant women followed from 20 weeks' gestation through 12 months postpartum. Neighborhood assessment of the study catchment area will include a street level inventory of general neighborhood characteristics and identification of all physical activity facilities, such as private and public facilities, parks, and trails. We will also add on two measures of physical activity among women during the postpartum period using accelerometry, in addition to self-report physical activity data already being collected. An extensive array of physiologic (e.g. anthropometric, sleep patterning, breastfeeding) and psychosocial (e.g. life events, social support, depression, anxiety) measures is already being collected. Advanced statistical techniques will be employed to accommodate multiple levels of influence on changes in physical activity. This proposal directly addresses the need to better understand multi-level factors involved in physical activity behavior change, RFA #CA-04-009 "Mechanisms of Physical Activity Behavior Change".